I borrowed this from KSR. If you rfactor is running fine then I wouldn't bother with these. If not then see if any of the below work.

Improving Performance: Utilising all cores in the CPURight click on the rfactor icon on your desk top. Select properties, then click on the Shortcut tab. In the target box add this +fullproc +highprioMine looks like this, but you will have rfactor installed in a different location no doubt."D:\Program Files\rFactor\rFactor.exe" +fullproc +highprio

This patches the rfactor.exe so it can use 4gb of virtual memory, instead of 2gb.Patches x86 executable s in order to let them have 4GB (instead of only 2) of virtual memory on x64 platforms. This lets rfactor use 4GB of RAM (on 64 bit machines with more than 4GB) rather than 2GB and can prevent CTDs, provided your machine and OS are both 64 bit and you have at least 4GB of RAM.

If everything's running fine, then excellent. if not, this is worth a try.

nVidia UsersSome nVidia GFX card users experienced some minor stuttering at sections of some tracks, even though frame rates were fine.Here is a link to a nVidia fix that might help.Installation: Put the included d3d9.dll in your rFactor root folder. (backup your orginal d3d9.dll file if you have one)